Explicitly enable implicit conversions as a language feature

Scala 2.10 introduced the idea of modularizing language features, eg
implicit conversions, and warning or erroring if lang features were
used without being explicitly enabled.

This project is largely based around the Java Git library JGit, so
implicit conversions are useful enough that I'm happy to enable them
everywhere.

http://docs.scala-lang.org/sips/pending/modularizing-language-features.html